1. Field of the Invention
The present invention relates to an image forming method, which is performed in apparatuses such as color copiers and color printers using an electrophotographic system.
2. Description of the Related Art
In conventional electrophotographic color image-forming apparatuses, such as color copiers and color printers, a single photoconductor drum is provided, on which respective toner images of colors such as yellow (Y), magenta (M), cyan (C), and black (BK) are successively formed. The toner image are then transferred to and superimposed on a recording sheet, and then thermally fixed onto the recording sheet to form a color image. In certain color image-forming apparatuses, respective toner images of colors such as in Y, M, C, and BK are successively formed on a photoconductor drum, then temporarily transferred to and superimposed on an intermediate transfer body, after which they are transferred to a recording sheet all at once, and thermally fixed onto the recording sheet to form a color image.
There are also color image-forming apparatuses that have different image-forming units corresponding to each of, for example, Y, M, C, and BK. Toner images of Y, M, C, and BK are each formed on the photoconductor drum of each image-forming unit successively and then transferred to and superimposed on a recording sheet; or temporarily transferred to and superimposed on an intermediate transfer body and then transferred to a recording sheet all at once. The toner images are then thermally fixed onto the recording sheet to form a color image.
In general, the color toners, which are to be transferred and fixed onto the recording sheet, are produced by dispersing or fusing and mixing colorants of a pigment, a dye or the like into a binder resin. The particle diameter of the toners is set at between several μm to several dozen μm. Such color toners are transferred to and superimposed on a piece of normal paper or coated paper such as general printing paper and then thermally fused and fixed onto the paper sheet.
A fixing device widely used in electrophotographic color copiers and printers is a roll fixing unit in which a fixing roll and a press roll are configured so as to be opposite each other. Fixation is performed between these rolls at a preset temperature under a preset load. The surface of the color image fixed in such a roll fixing unit has an irregularity of between 10 to 100 μm, which results from toner layer fluctuation and causes unevenness in gloss. Such color images formed on normal paper or coated paper reflect light irregularly and when viewed with the naked eye, exhibit poor in glossiness.
In light of this problem, Japanese Patent Application Publication (JP-B) Nos. 04-31389 and 04-31393 disclose a belt fixing unit for producing high glossiness, in which a sheet is placed on the surface of a toner image on a support after the fixation in an image forming apparatus, then heated and pressed so that the toner image is fused again, and then cooled and peeled.
These proposed techniques can improve gloss development, however, cannot produce both uniform high gloss image or flat images, not to mention photorealistic images.
The prior art for producing photorealistic images in the electrophotographic system includes a method wherein a transfer sheet that comprises a substrate and a 50–100 μm thick transparent resin layer formed on the substrate is used. The toner image is embedded into the transparent resin layer with a heat roll fixing unit so that irregularities in the toner layer can be reduced and the glossiness of the color image can be improved. Japanese Patent Application Laid-Open (JP-A) No. 05-127413 discloses a method wherein the toner image is transferred to an image transfer sheet that comprises a transparent resin layer of a tetrahydrofuran-soluble crosslinked resin with a glass transition temperature of 40 to 70° C.; the toner image is embedded into the transparent layer by means of a belt-type fixing unit. Additionally, JP-A Nos. 05-216322, 06-11982 and 2002-91048 disclose methods where the toner image is transferred to an image transfer sheet comprising a thermoplastic resin coating, and embedded into the transparent resin layer with a belt-type fixing unit.
The technologies disclosed in these publications involve fixing the color toner image onto the transfer sheet by pressing the color toner image with a heat roll so that the image is heated and fused into the transparent resin layer of the transfer sheet surface. Such processes can provide a color image with little surface irregularity and reduced diffuse light reflection, thereby producing a high-quality color image with no fluctuation in gloss.
Nonetheless, in the methods disclosed in JP-A Nos. 05-127413, 05-216322, 06-11982, and 2002-91048, if the recording sheet shrinks significantly during the heating and fixing process in the belt-type fixing unit, adhesion between the recording sheet and the belt-type fixing unit can be reduced, causing the recording sheet to easily peel off from the fixing unit. Accordingly, these processes have the potential to cause deterioration of high glossiness or uniform gloss on the toner image-receiving layer and the image portion. This in turn can cause peeling of the recording sheet before the separation point, causing jamming of the recording sheet in the apparatus.